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.Dd $Mdocdate: November 2 2019 $
.Dt CMS_COMPRESS 3
.Os
.Sh NAME
.Nm CMS_compress
.Nd create a CMS CompressedData structure
.Sh SYNOPSIS
.In openssl/cms.h
.Ft CMS_ContentInfo *
.Fo CMS_compress
.Fa "BIO *in"
.Fa "int comp_nid"
.Fa "unsigned int flags"
.Fc
.Sh DESCRIPTION
.Fn CMS_compress
creates and returns a CMS
.Vt CompressedData
structure.
.Pp
.Fa comp_nid
is the compression algorithm to use or
.Dv NID_undef
to use the default algorithm.
Currently, the default algorithm
.Dv NID_zlib_compression
is the only supported algorithm.
If zlib support is not compiled in,
.Fn CMS_compress
always returns an error.
.Pp
.Fa in
provides the content to be compressed.
.Pp
Any of the following flags (OR'ed together) can be passed in the
.Fa flags
parameter:
.Bl -tag -width Ds
.It Dv CMS_TEXT
Prepend MIME headers for type text/plain to the data.
.It Dv CMS_BINARY
Do not translate the supplied content into MIME canonical format,
even though that is required by the S/MIME specifications.
This option should be used if the supplied data is in binary format.
Otherwise, the translation will corrupt it.
If
.Dv CMS_BINARY
is set,
.Dv CMS_TEXT
is ignored.
.It Dv CMS_STREAM
Return a partial
.Vt CMS_ContentInfo
structure suitable for streaming I/O: no data is read from
.Fa in .
Several functions including
.Xr SMIME_write_CMS 3 ,
.Xr i2d_CMS_bio_stream 3 ,
or
.Xr PEM_write_bio_CMS_stream 3
can be used to finalize the structure.
Alternatively, finalization can be performed by obtaining the streaming
ASN1
.Vt BIO
directly using
.Xr BIO_new_CMS 3 .
Outputting the contents of the
.Vt CMS_ContentInfo
structure via a function that does not
properly finalize it will give unpredictable results.
.It Dv CMS_DETACHED
Do not include the compressed data in the
.Vt CMS_ContentInfo
structure.
This is rarely used in practice and is not supported by
.Xr SMIME_write_CMS 3 .
.El
.Pp
Additional compression parameters such as the zlib compression level
cannot currently be set.
.Sh RETURN VALUES
.Fn CMS_compress
returns either a
.Vt CMS_ContentInfo
structure or
.Dv NULL
if an error occurred.
The error can be obtained from
.Xr ERR_get_error 3 .
.Sh SEE ALSO
.Xr CMS_ContentInfo_new 3 ,
.Xr CMS_uncompress 3
.Sh STANDARDS
RFC 3274: Compressed Data Content Type for Cryptographic Message Syntax (CMS)
.Sh HISTORY
.Fn CMS_compress
first appeared in OpenSSL 0.9.8h
and has been available since
.Ox 6.7 .
.Pp
The
.Dv CMS_STREAM
flag first appeared in OpenSSL 1.0.0.
